Methanol carbonylation, the reaction of methanol with carbon monoxide, is used to produce a significant share of the world's acetic acid and represents the basis for virtually all new acetic acid capacity. The fundamental process, whereby methanol and carbon monoxide are reacted in the presence of a rhodium catalyst and methyl iodide promoter, is disclosed in U.S. Pat. No. 3,769,329 and has become well-known as the "Monsanto process". Although numerous improvements have since been developed, the use of an iodine-containing promoter, either as an organic iodide or metal iodide salt, has proven necessary to obtain industrially-competitive reaction rates and production economies.
Unfortunately, the use of any suitable iodine-containing promoter invariably results in the incorporation of trace iodine and organic iodide impurities into the final acetic acid product. These contaminants result from numerous transformations (thermal cracking, recombination, isomerization, etc.) of the iodine-containing catalyst promoters which occur not only in the reactor but also in downstream equipment, such as distillation column reboilers and recycle lines. A resulting array of C.sub.1 to C.sub.10 organic alkyl iodide species is produced, which are removed from the acetic acid product with varying degrees of effectiveness via the standard distillation steps used in downstream purification. Additionally, iodine may be present in the acetic acid product in the form of hydrogen iodide or iodide salts. Ultimately, without supplemental treatment to remove trace iodine-containing contaminants, product acetic acid made using methanol carbonylation technology with even the most careful fractionation steps, will still contain a small amount, typically below 100 parts per billion (ppb) of total iodine (both organic and inorganic) by weight.
The interest in a process for essentially complete removal of iodine-containing contaminants from acetic acid stems from the large share (about 40 to 50%) of its use as a precursor for vinyl acetate monomer (VAM) synthesis. Current methods of VAM production rely on a catalyst which is intolerant to even minute levels of iodine-containing compounds in the acetic acid feedstock. Therefore, the VAM production costs associated with reduced catalyst life increase dramatically with increasing feed iodine concentration.
Several disclosures in the prior art present techniques for the selective removal of iodine-containing species from process streams such as nuclear reactor containment environment off gases as well as emissions from spent nuclear fuel reprocessing operations. For example, U.S. Pat. No. 3,658,467 addresses the removal of radioactive iodine-containing materials from the gaseous waste streams generated either during normal nuclear fuel reprocessing operations or even in the event of a fuel element cladding failure whereby radioactive methyl iodide is formed in significant amounts. The solution proposed in the '467 patent is a zeolite X molecular sieve exchanged with silver for treating the gaseous waste stream. All cited examples referring to the adsorptive ability of this formulation are based on performance in a dry air stream contaminated with trace radioactive methyl iodide. The structures of X-type zeolites are known to have aluminosilicate frameworks with maximum silica to alumina molar ratios, expressed as the ratio of SiO.sub.2 to Al.sub.2 O.sub.3 in the fundamental zeolite framework, of about 3 and pore openings typically in the range of 7 to 8 .ANG..
In U.S. Pat. No. 4,735,786, an alternate solution for filtering radioactive iodine-containing compounds from nuclear facility exhaust gases in the event of an accident is proposed. In offering an improvement over the prior art, the '786 patent recognizes the practical deficiencies of silver-exchanged zeolite X adsorbent for this service under high humidity conditions. The improvement offered is a different type of adsorbent, characterized as a high silica to alumina molar ratio pentasil zeolite. The adsorbent specified is exemplified by the well-known ZSM-5 type material, which is clearly described in U.S. Pat. No. 3,702,886 as having ten-member rings forming medium-sized pores in the range of 5.1 to 5.6 .ANG.. The teachings and specific examples of the '786 patent are restricted to pentasil zeolites having silica to alumina molar ratios in the range of 15 to 100, preferably 20 to 50.
In U.S. Pat. No. 4,913,850, another solution for methyl iodide removal from gaseous streams is presented, whereby a silver-exchanged "binderless" zeolite material, composed of 80 to 90% zeolite X and 10 to 20% zeolite A, is used. Among the possible candidates for zeolite X materials, those having the faujasite structure are of particular interest. As mentioned previously, zeolite X formulations generally have a maximum silica to alumina molar ratio of 3. In U.S. Pat. No. 5,075,084, the progress of treating radioactive iodine-containing gas streams is continued, where the problem of the proposed silver-exchanged zeolite material catalyzing the highly exothermic reaction of hydrogen and oxygen and, in the extreme case, causing catalytic ignition of hydrogen, is solved. According to the '084 patent, this undesired side reaction is suppressed when a heavy metal such as lead is added to the silver-exchanged adsorbent. The underlying zeolite compositions of the preferred materials in this patent and the previously-mentioned '850 patent are identical.
In U.S. Pat. No. 4,088,737, gaseous radioactive methyl iodide removal is further addressed in a multi-step treatment procedure where the initial gas purification is performed with a silver-exchanged zeolite exemplified by zeolite X. After iodine-compound breakthrough, regeneration and concentration steps are undertaken, which involve i) withdrawing the spent adsorbent from contact with the gaseous waste stream, ii) subjecting the adsorbent to desorption conditions with a hydrogen-rich stream to produce a hydrogen iodide containing off gas, and iii) treating this effluent gas with a lead-exchanged zeolite to re-adsorb and concentrate the desorbed hydrogen iodide. Lead-exchanged zeolite X is specifically cited as achieving the desired result for the final adsorption step. The advantage of the multi-step treatment is that the long-term storage of the contaminated material is less expensive for the lead-exchanged zeolite, compared to a silver-exchanged material.
In spite of these continuing developments and improvements in trace iodine and organic iodide removal from gaseous effluent streams, the methods employed have been found unsuitable for the more difficult problem of iodine-containing compound adsorption from corrosive liquids, such as commercial acetic acid product streams. Adsorbent carrier materials of the prior art such as zeolite X and zeolite A, which are classified as having low silica to alumina molar framework ratios (typically below 5), have experimentally been proven to be unstable in acetic acid. This means that the dissolution (or leaching) rate of framework components into the liquid is sufficiently large to render such materials ineffective for iodine-containing compound adsorption service in corrosive liquid media.
Depending on the specific silica to alumina framework molar ratio, the pentasil zeolites, exemplified in '786 patent for gas-phase iodine-containing compound removal, are significantly more stable in acetic acid than zeolite types X and A. However, the high silica to alumina framework molar ratios (20 to 50) of the preferred zeolites of the '786 patent have a significantly lower capacity for iodine-reactive metal by ion exchange than the crystalline manganese phosphate materials of the present invention. As is known in the art, increasing the silica to alumina framework molar ratio of a zeolite directionally improves its resistance to decomposition in corrosive environments, at the expense of reduced ion-exchange capacity. The reported silver loadings for the pentasil zeolite formulations in the '786 patent examples were below 10% of the total adsorbent weight. In contrast, the improved metal loadings (maximally 50% by weight) achieved by using metal-exchanged crystalline manganese phosphates result in overall greater capacity for iodine-containing compound removal, expressed in milligrams of iodine per gram of adsorbent.
Other prior-art teachings more specifically apply to iodine-compound removal from corrosive liquid media, where the principal area of concern, as described previously, is in the manufacture of carboxylic acids such as acetic acid via a process which results in a product stream contaminated with trace amounts of iodine-containing byproducts. Thus far, techniques such as adsorptive distillation, iodine scavenger addition, alkyl iodide oxidation to molecular iodine, and others have not achieved practical utility, because such methods not only fail to achieve the extremely low levels of iodine-containing compounds demanded industrially but also require additional purification steps. For this reason, far greater emphasis has been placed on the development of solid materials capable of adsorbing essentially all iodine-containing compounds from acetic acid streams.
For instance, in U.S. Pat. No. 5,457,230, the use of activated carbon fiber is contemplated for this purpose. However, the examples demonstrate the removal of molecular iodine and hydrogen iodide only and fail to specifically disclose the level of iodine-containing compounds in the treated acetic acid stream. In the case of iodine-compound removal from acetic acid, it is the ability of the invention to provide a treated product with only extremely minute levels of total iodine which primarily determines its practical utility. It is known in the art that activated carbon alone can neither remove iodine-containing compounds from commercial acetic acid streams to single parts per billion levels, nor can it effectively remove organic iodide species, such as methyl iodide and hexyl iodide which are commonly present in these product streams, without the use of an iodine-reactive metal.
Recently, considerable development efforts in acetic acid purification technology have focused on resins containing iodine-reactive metals such as silver, mercury, copper, lead, thallium, palladium or combinations of these metals known to react with iodine-containing compounds to form insoluble complexes. For example, in U.S. Pat. No. 4,615,806, the removal of these impurities is achieved with a macroreticulated strong acid cation-exchange resin which is stable in the organic medium and has at least one percent of its active sites converted to the silver or mercury form, presumably by cation-exchange. The use of macroreticulated resins is claimed as an advance over the prior art formulations, which are generally characterized as gel-type ion-exchange resins, for this service. In U.S. Pat. No. 5,139,981, other silver-exchanged resins are offered, along with a novel technique for preparing such resin compositions. In U.S. Pat. No. 5,220,058, a performance benefit is claimed, whereby the subject resin contains thiol functional groups, compared to the prior art sulfonate functional groups, which are exchanged with the iodine-reactive metal. In U.S. Pat. No. 5,227,524, the resin degree of crosslinking is decreased somewhat, resulting in improved silver utilization. In U.S. Pat. No. 5,300,685, the iodine-reactive metal is coordinated, as a salt, with a polymeric resin, rather than being ionically-bound to a cation-exchange resin. In U.S. Pat. No. 5,344,976, a resin guard bed without the iodine-reactive metal is placed upstream of the metal-exchanged resin to scavenge any metal cations in the acetic acid stream which would otherwise potentially displace the iodine-reactive metal. Finally, in U.S. Pat. No. 5,801,279, an improved method of operating the iodine-compound removal step is disclosed in order to reduce the amount of leaching of the iodine-reactive metal into the treated acetic acid effluent stream. As noted in this reference, the dissolution of the iodine-reactive metal is acknowledged as a problem for iodine-compound removal techniques of the prior art whereby metal-exchanged resins are applied.
While the invention of the U.S. Pat. No. 4,615,806 patent and other modified resin-based formulations have been used commercially with some success, resins in general suffer some disadvantages, in addition to the previously-mentioned metal loss, when used in the acetic acid environment of the present invention. More specifically, resins, even those characterized as "stable" are known to "swell" or increase in diameter by as much as 50% when exposed to an organic medium, making bed design difficult. Resins are also vulnerable to decomposition at relatively mild conditions and are furthermore susceptible to chemical attack by corrosive reagents. These factors additionally complicate the use of a resin-based material for the purification of acetic acid.
The problem therefore addressed by the present invention is to provide an adsorbent for use in removing iodine-containing compounds from commercial acetic acid feed streams where the adsorbent is free of the chemical exposure effects and swelling problems associated with the typical organic materials used in the prior art. There are significant teachings in the prior art associated with the use of non-resin adsorbents that point away from their utility in this treatment service. In particular, in the comparative example recited in U.S. Pat. No. 4,615,806 (column 6, lines 36 to 49), a silver-exchanged zeolite, characterized as 1/16 inch 5A molecular sieve pellets, was tested in acetic acid for contaminant methyl iodide removal and found to be unstable as evidenced by the continuous silver leaching from the adsorbent and the finding of a yellowish precipitate in the treated effluent. Given this discouraging result, it is remarkable that a suitable inorganic adsorbent for use in this corrosive environment has been discovered.
The adsorbent material of the present invention in fact comprises a crystalline manganese phosphate which has been cation exchanged with a metal known to react with iodine-containing compounds. The crystalline manganese phosphate composition is represented by the empirical formula: EQU (A.sup.a+).sub.v (Mn.sup.b+)(M.sup.c+).sub.x P.sub.y O.sub.z
where A is typically either an alkali or alkaline earth metal, but may also be hydronium ion, ammonium ion, organoammonium ions, silver, copper (II), zinc (II), nickel (II), mercury (II), or cadmium (II), or any combination of these species, "a" represents a weighted average valence of A and varies from about 1.0 to about 2.0, "v" is the mole ratio of A to Mn and varies from about 0.1 to about 10.0, "b" is the average valence of Mn and has a value of greater than 2 to about 4, M is (optionally) a metal selected from the group consisting of Al, Fe.sup.3+, Ga, Sn.sup.4+, Ti, Sb.sup.5+, Ag, Zn, Cu, Ni, Cd, and mixtures thereof, "x" is the mole ratio of M to Mn and varies from 0 to about 3.0, "c" is the weighted average valence of M and varies from about 1.0 to about 5.0, "y" is the mole ratio of P to Mn and varies from about 0.1 to about 8.0, and "z" is the mole ratio of O to Mn and has a value determined by the equation: EQU z=1/2(a.cndot.v+b+x.cndot.c+5.cndot.y).
Of the crystalline manganese phosphate compositions of this type, those characterized as having manganese with an average valence of greater than 3.0 but less than or equal to 4.0 are defined and described according to their unique structural details and synthesis procedures in U.S. Pat. No. 5,780,003, which is incorporated by reference. The broader range of crystalline manganese phosphate compositions applicable to the present invention include structures with manganese having an average oxidation state of greater than 2.0 but less than or equal to 4.0. All suitable materials are characterized, as described in the '003, as having an extended network. By extended network is meant that the defining Mn--P--O structural unit of the material repeats itself into at least two adjacent unit cells without termination of bonding, i.e., the material is not molecular.
The crystalline manganese phosphates of the present invention are further characterized in that they exhibit ion-exchange capacity with metals (e.g. silver) which are reactive with iodine and iodine-containing compounds. Ion-exchanged crystalline manganese phosphates alone are therefore useful as adsorbents of the present invention, but it is often desired to bind such compositions into larger particles of various shapes using an inorganic refractory metal oxide binder (e.g. silica). It is also possible to ion-exchange crystalline manganese phosphates with an effective amount of iodine-reactive metal subsequent to binding. Even after use in the iodine-containing compound adsorption service of the present invention, adsorbents comprising ion-exchanged crystalline manganese phosphate compounds can be reactivated using a relatively simple metal re-exchange procedure.